1. Field of the Invention
The invention relates generally to wireless communication systems and, more particularly, to reliable decoding of page indicators in a quick paging channel.
2. Description of the Related Art
In a typical wireless communication system such as a cellular radio frequency (RF) radiotelephone system, a base station communicates with a mobile station, operating within an area served by the base station system.
Multiple access wireless communication between the base station and the mobile station occurs via RF channels that provide paths over which communication signals such as voice, data, and video are transmitted. Base-to-mobile station communications are said to occur on a forward- or down-link channel, while mobile-to-base station communications are referred to as being on a reverse- or up-link channel.
Code division multiple access (CDMA) is one example of a well-known digital RF channel technique. In general, all users of a CDMA system transmit and receive communications signals over a common frequency spectrum, and specialized codes, such as Walsh codes, are used for separating multiple communication signals from one another in the system. Each specialized code typically represents one communication channel within the common frequency spectrum. A communication channel may be, among other things, a paging channel or a traffic channel.
The IS-2000 standard provides for paging channels divided into time slots, or paging channel time slots. According to the IS-2000 standard, one paging channel may be used by the base station. Within those paging channels, a plurality of paging channel time slots are assigned to the mobile station. A base station transmits pages and other messages to a mobile station in the paging channel time slots assigned to that mobile station.
The IS-2000 standard further provides for quick paging channels. Under IS-2000, each base station has one quick paging channel. Quick paging channels, like paging channels, are divided into time slots, or quick paging channel time slots. When a mobile station is to receive a message on a paging channel, the base station enables one or more page indicators on the quick paging channel to alert the mobile station that it should monitor its assigned paging channel. This alleviates the requirement that a mobile station monitor every assigned time slot of the paging channel for messages. Instead, the mobile station may enter a sleep mode, waking only to monitor its assigned page indicators on the quick paging channel. For a mobile station to operate in an idle mode, the base station transmits to the mobile station at specific time slots only on the paging channel in the IS-95 standard. In the IS-2000 standard, however, the quick paging channel is also divided into 80 ms time slots, and specific slots are assigned to a given mobile station. In between two of such slots, a mobile station can go to sleep. This mode of operation is called a slotted mode.
For a mobile station in a slotted mode, if the page indicator in its time slot of the quick paging channel is on, the mobile station will decode the message in the next time slot of the paging channel. Otherwise, the mobile station may return to the sleep mode until the time of its next scheduled page indicator. Because the page indicators are relatively short, the mobile station conserves energy by remaining in the sleep mode for longer periods of time. Consequently, the mobile station's battery life is increased.
The purpose of the quick paging channel is that a mobile station in a slotted mode while in idle state only needs to wake up and decode the page indicators instead of an entire paging channel slot message. Since the quick paging channel contains single-bit messages without coding, its takes less power to process than the paging channel message. Power saving is expected by exploiting the quick paging channel efficiently. Typically, this depends heavily on the detection accuracy of page indicators. If page indicators are detected to be on when they are actually off, there is a false alarm, and the mobile station unnecessarily wakes up. On the other hand, if page indicators are detected to be off when they are actually on, the mobile station sleeps through the message and thus misses it.
There is therefore a need for a reliable decoding scheme for avoiding both of these cases in order to achieve longer standby time without missing calls.